Tuesday, February 18, 2014

A paper published today in the Journal of Geophysical Research Atmospheres finds a significant increase in mid-latitude clouds over the past 10 years. According to the authors, " Increases in cloud frequency appear to be driven by corresponding temperature decreases over the same time period. During this time, solar activity decreased from an active to a quiet period, which might have been partially responsible for the temperature decrease over this time period."

Motivated by numerous ground-based Noctilucent Cloud (NLC) sightings at latitudes as low as ~ 40oN in recent years, we have conducted a study to determine if there have been any systematic NLC increases in the mid-northern latitudes. This question is addressed through investigating both measured and modeled Polar Mesospheric Cloud (PMC) occurrence frequencies. Temperature measured by the Soundings of the Atmosphere using Broadband Radiometer (SABER) instrument on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite over the 2002-2011 time period and a 7-year water vapor climatology developed from data measured by the Microwave Limb Sounder (MLS) instrument on the Aura satellite for the 2005-2011 period, are used to simulate mid-latitude PMCs. PMCs measured by the Odin Optical Spectrograph and InfraRed Imager System (OSIRIS) and the Space Test Program Satellite-1 (STPSat-1) Spatial Heterodyne Imager for Mesospheric Radicals (SHIMMER) instruments are used to extensively validate the model generated PMC results. After validating the model against PMC data, the model results were used to examine changes in PMCs at mid-latitudes between 2002 and 2011. Results show a statistically significant increase in the number of PMCs [Polar Mesospheric Clouds] each season in the latitude range 40o-55oN for the past ten-year period examined. Increases in cloud frequency appear to be driven by corresponding temperature decreases over the same time period. During this time, solar activity decreased from an active to a quiet period, which might have been partially responsible for the temperature decrease over this time period.